The main body or door of a refrigerator is provided with a lining material produced from a sheet of a styrene-based resin such as an acrylonitrile/butadiene/ styrene copolymer (ABS) by vacuum forming or pressure forming or a combination thereof (those forming methods are hereinafter referred to as "differential pressure forming"). This lining material serves to prevent corrosion of a frame of stainless steel or steel which is coated with a paint followed by baking, and also provides projections necessary for holding shelves. An attempt has been made to use a lining material made of a polypropylene having an excellent surface luster and a low degree of water absorption in place of the lining material made of ABS.
In order to increase the heat-insulating properties of a refrigerator, it has been proposed to provide a urethane foam sheet between the stainless steel plate having a baked coating of the frame and the lining material. In producing such a laminated structure, when a polyolefin-based resin sheet is used as a lining material, adhesion between the polyolefin-based resin sheet and the urethane foam sheet as a heat-insulating material must be strong. This is achieved by, for example, laminating the lining material and urethane foam sheet which had previously been subjected to differential pressure forming, with a liquid acryl or epoxy-based adhesive, thereby producing a laminated member and then press bonding the laminated member to the inner surface of the stainless steel frame having a baked coating which had been coated with an adhesive. This procedure, however, will increase production cost since it involves a plurality of steps. For this reason, it is preferred to employ an in situ fabrication method as shown in FIG. 1 that a foamable urethane solution 4 is injected into a cavity 3 formed between a mold frame 1 and a lining material 2 and then expanded and hardened to produce a composite structure that the mold frame 1, the urethane foam 4' and the lining material 2 are combined together in one body.
Other possible method is a method comprising coating an organic solvent type adhesive on a polyolefin resin sheet, drying the adhesive and bonding the sheet to a urethane foam of a heat-insulating material. However, use of the organic solvent is not preferred from the standpoints of operation environment such as danger of fire or worker's health.
Prior to differential pressure forming of the polyolefin-based resin sheet, it is necessary to remove dust attached onto the surface thereof with cloth, for example. This problem can be overcome by introducing an antistatic agent into the sheet in the course of production thereof. In this method, however, the desired antistatic capability can be obtained only in 30 to 50 days after molding of the sheet. During this period, it is undesirably necessary to pay attention so that the sheet is not affected by dust.
The present inventors have made extensive investigations on a coating type adhesive composition which satisfies the above described properties required in the in situ production of a laminated structure insulating material of urethane foam and ABS, or urethane foam and polypropylene, and have provided a composite heat-insulating material having the structure that a molded product of a polyolefin-based resin or styrene-based resin and a urethane foam are laminated with an adhesive containing the following Components (A) and (B) (Japanese patent application Nos. 136344/83 and 206807/83 corresponding to U.S. Pat. No. 4,500,597).
Adhesive Composition:
(A) 20 to 80% by weight of a water-soluble polyimine compound selected from the group consisting of polyethyleneimine, poly(ethyleneimine-urea) and an ethyleneimine adduct of polyaminepolyamide;
(B) a water-soluble polymer comprising an amphoteric compound derived by modification with an amphoterizing agent of a tertiary nitrogen atom of a polymer comprising the following Components (a) to (c) having an antistatic property: ##STR1##
(c) 0 to 20% by weight of other hydrophobic vinyl monomer,
(wherein R.sup.1 is H or CH.sub.3, R.sup.2 and R.sup.3 are each H or an alkyl group having from 1 to 2 carbon atoms, R.sup.5 is an alkyl group having from 1 to 18 carbon atoms, and A is an alkylene group having from 2 to 6 carbon atoms).
The composite heat-insulating material was such that the urethane foam was strongly bonded to the polyolefin-based resin sheet or styrene-based resin sheet, and could be sufficiently and practically used as the inner decorative material of a refrigerator.
However, it was found that the products wherein the adhesive strength of the corner portions of the resin sheet vacuum formed is lower than the other portion thereof are present in the proportion of 10% or less.